Problem: Ten families have an average of 2 children per family. If exactly two of these families are childless, what is the average number of children in the families with children? Express your answer as a decimal to the nearest tenth.
Explanation: There are $10(2)=20$ children total.  If $2$ families are childless, $8$ have children.  So the average number of children for a family with children is

$$\frac{20}{8}=\boxed{2.5}$$